A number of synthetic crystalline zeolites have previously been prepared in the past. The most prominent and much exploited among them is zeolite Y. This zeolite is a synthetic analog of naturally occurring zeolite named faujasite, with respect to structure. Prior-art work on zeolite Y is related to synthesis with different types of raw materials followed by improvement in silica to alumina ratio of the framework.
U.S. Pat. No. 2,882,243 awarded to Milton and U.S. Pat. No. 3,130,007 awarded to Breck describes a process for the synthesis of Y type zeolite in which aluminum and silicon salts used are soluble in aqueous medium.
In general, crystalline molecular sieves such as zeolite A, zeolite Y, zeolite X, mordenite, ZSM series zeolites are synthesized from a reaction mixture comprising soda, alumina and silica in an aqueous media. In the process of formation of molecular sieves, only part of soda and silica are consumed, while major amount of alumina is utilized. Thus, major part of silica and soda goes unutilized in the form of the mother liquor. Improvement in the process for effective utilization or reuse of the mother liquor obtained after the synthesis of zeolite has drawn scanty attention. This mother liquor being unbalanced in composition permits only partial recycling for preparing a fresh batch of molecular sieves.
In some of the prior art processes for the manufacture of zeolite Y and X, partially acidic aluminum sulfate as a aluminum source is used. During the preparation of precursor gel, sodium sulfate byproduct is formed by the reaction between sodium hydroxide and soda of silicates with aluminum sulfate. This sodium sulphate byproduct remains in the mother liquor even after recovery of crystallized molecular sieves and act as a hurdle, and disallows even partial recycling of mother liquor for preparing a fresh batch of molecular sieves.
U.S. Pat. No. 2,882,244 to Milton discloses the preparation of type X zeolite by a process, wherein it is suggested that the mother liquor may be reused after enrichment with proper amounts of reactants to give a properly proportioned reaction mixture.
U.S. Pat. Nos. 3,574,538 and 3,808,326 to McDaniel, U.S. Pat. No. 3,671,191 to Maher et al and U.S. Pat. No. 3,789,107 to Elliott disclose methods for preparing zeolites wherein zeolite nucleation centers are combined and reacted with sources of silica, alumina, sodium hydroxide and water.
U.S. Pat. No. 3,639,099 granted to Elliott, refers to a process for preparation of Y type zeolite from sodium aluminate and sodium silicate and effective utilization of raw materials. In this invention unreacted silica has been precipitated with a mineral acid and reused as a source of silica for further synthesis of molecular sieves.
U.S. Pat. No. 3,898,319 granted to Weber discloses a method for utilizing waste liquor obtained during synthesis of type Y zeolite, wherein the excess silicate present in the mother liquor is recovered as solid amorphous silica by precipitation with carbon dioxide. The precipitated silica is then reused to prepare additional type Y zeolite.
U.S. Pat. No. 3,939,246 teaches a process for crystallization of aluminosilicate zeolites of the molecular sieve type either as such or in aggregate combination with clay mineral accomplished by the addition of a flux constituting an alkali metal salt to a kaolin-type clay prior to calcination and caustic aging of said clay.
U.S. Pat. No. 4,016,246 teaches a process for preparing faujasite zeolite employing colloidal silica as a silica source.
U.S. Pat. No. 4,164,551 teaches a process for the preparation of type Y zeolite by reacting silica, alumina, sodium hydroxide and water to produce a slurry of type Y zeolite and excess silicate containing mother liquor. Silicate is recovered from the mother liquor as precipitated silica/alumina hydrogel by the addition of an acid aluminum salt such as aluminum sulfate. The precipitated silica/alumina hydrogel is reused as a source of silica and alumina for preparing zeolite.
U.S. Pat. No. 4,175,059 teaches a process for preparing a synthetic faujasite having a novel platelet-type crystalline shape and silica to alumina ratio above 2.2 by adding potassium ions to a seeded faujasite synthesis slurry and heating to convert to the synthetic faujasite. The use of the novel zeolite form as a catalyst promoter and an adjuvant for strengthening formed zeolite such as beads, balls, pills and extrudates is also disclosed.
U.S. Pat. No. 4,178,352 refers to a process for preparing type Y zeolite using a minimum excess of reactants by a method wherein required sodium hydroxide, silica, alumina and water reactants are combined in multi-stage procedure to obtain a uniform fluid reaction slurry. The procedure permits the efficient commercial production of high quality, type Y zeolite and minimizes the formation of excess silicate containing by-product effluent.
Disposal of considerable quantities of silica, soda and sodium salts present in the mother liquor represents both a considerable economic waste and a substantial pollution control problem. Previous attempts to recycle the excess sodium silicate solution to a type Y zeolite synthesis process have not been particularly successful in that excess water and soda/sodium salts associated with the mother liquor tends to produce inferior or no zeolite product.
U.S. Pat. No. 4,228,137 relates to a process for an improvement in the production of zeolites, particularly, zeolites of the faujasite type, employing clay based seeds derived from natural halloysite.
U.S. Pat. No. 4,235,753 relates to an improved process for the production of mechanically strong shaped crystalline zeolite aluminosilicate bodies from precursor bodies composed of kaolin clay calcined at elevated temperature, the crystallized bodies having essentially the same size and shape as the precursor bodies.
U.S. Pat. No. 4,931,267 refers a faujasite polymorph having a silica to alumina ratio greater than 6, and containing tetrapropyl ammonium and/or tetrabutyl ammonium trapped within the supercages of said structure.
U.S. Pat. No. 5,385,717 refers to the preparation of faujasite type structure from aluminosilicate gel containing a structuring agent ST.
U.S. Pat. No. 6,027,708 relates to a process for the production of fly ash based Zeolite-Y (FAZ-Y).
U.S. Pat. No. 6,284,218 refers to a process for the preparation of large crystallite size, highly crystalline faujasite type zeolite by heating a mixture of sodiumaluminosilicate gel and seed. In this invention, said sodiumaluminosilicate gel was prepared by reacting aluminum sulfate with a mixture of sodium silicate and sodium hydroxide.
U.S. Pat. No. 6,299,854 refers to a method of producing artificial zeolite, wherein heat treatment is performed in an alkali solution on a mixture obtained by adding at least one of cullets of glass waste, diatomaceous earth and aluminum dross to incineration ash of combustible waste.
In all the prior art processes for the preparation of faujasite type zeolite silica has been sourced from one of the raw materials such as sodium silicate, clays, colloidal silica, precipitated silica, gel silica, fumed silica including silicas such as those known by trade names as “santocel”, “Cab-o-sil”, ‘hi-Sil”, “QUSO” and “Ludox-AS 40”. While alumina has been sourced from aluminum salts such as sodium aluminate, aluminum sulfate, pseudoboehmite alumina, gel alumina, clays etc. Depending upon sources chosen for silica and alumina, required amount of soda in the form of sodium hydroxide has been taken.
It may be seen from the various examples for composition of reaction mixtures used in prior art processes for the synthesis of faujasite type zeolite, that soda has been used in moles ranging 2-14, while silica is used in moles ranging from, 3-50 on the basis of the use of one mole of alumina. Water content ranges from 100-1000 moles.
Similarly, from various examples sited in the prior art processes, composition of faujasite type zeolite has been expressed in moles, on volatile free basis as, 0.9-1.05 Na2O, 1 Al2O3, 3.5-6 SiO2.
From various published literatures and from the yield pattern and composition for zeolites it can be seen that, alumina being highly active, major portion of this species present in the reaction mixture is consumed in the building of zeolite framework. It may be seen that, core portion of most of the zeolites are rich in alumina, while the surface is alumina deficient.
Further, it may be concluded that, as availability of alumina in the reaction mixture become scarce, zeolite stops growing. At this stage, crystallized product is recovered by filtration. Significant amount of silica and soda still remain in the mother liquor. Further, it may be seen from the composition of zeolites, that utilization of soda in building of zeolite is the least, and thus larger portion of it remains in the mother liquor. Presence of excess soda in the mother liquor, permits only partial recycling for fresh zeolite reaction mixture.
In example IV, of U.S. Pat. No. 3,130,007, composition of starting reaction mixture was,
9 Na2O:Al2O3:12 SiO2:314 H2O,
While, the molar composition of crystallized product is,
0.92 Na2O:Al2O3:3.29 SiO2:7 H2O.
From the above mentioned molar compositions for reaction mixture and zeolite, by assuming all the alumina present in the former is fully utilized, it can be concluded that, only 0.92 moles out of 9 moles of soda present in the reaction mixture is consumed. Similarly, 3.29 moles out of 12 moles of silica present in the reaction mixture is consumed. Mother liquor having unutilized soda and silica is sent to effluent treatment plant, thus causing economic loss.
Similarly, in example 2 of U.S. Pat. No. 6,284,218, the molar composition of reaction mixture has been worked out as:
3.94 Na2O:2.85 Na2SO4:Al2O3:10.92 SiO2:188.89 H2O,
While the composition of zeolite on anhydrous basis is:
Na2O:Al2O3:5.8 SiO2.
With the assumption, all the alumina has been utilized in the reaction, composition of mother liquor on anhydrous basis, after the recovery of zeolite has been worked out as,
2.94 Na2O:2.85 Na2SO4:5.12 SiO2.
In this case, presence of 2.85 moles of Na2SO4 in the mother liquor, makes it unfit for even partial recycling. Hence, entire mother liquor has to be sent to effluent treatment plant.
Looking at the valuable amount of silica present in the mother liquor and the same being sent to effluent treatment plant as referred to in this prior art citation, it is thought worth to recycle this material, after eliminating undesired sodium salt and soda.